Diseases in the vascular system are still the leading cause of mortality and morbidity in developed countries despite considerable therapeutic progress in recent years. Blood flow velocity provides critical information needed for the diagnosis of vascular diseases, planning of interventional surgery treatment, and monitoring of endovascular treatment of brain arteriovenous malformations. The lack of such flow characteristics prevents understanding the underlying hemodynamics and its correlation with multiple cerebrovascular diseases. Therefore, there is a critical need to obtain precise blood flow velocities in the vascular system, which can be used to estimate the blood pressure, wall shear stress on the arterial wall and other hemodynamic indicators, and aid in diagnosing and treating a host of vascular diseases. Current diagnostic tools suffer from poor accuracy or low spatial and temporal resolutions. To overcome this limitation, we propose to develop an ultrafast, high-resolution X-ray blood flow velocimetry system that will provide quantitative blood velocity maps in the endovascular system. Furthermore, researchers using numerical simulations to understand the hemodynamics desperately seek detailed blood velocity and stress measurements for comparison and validation of their computer codes. The proposed project aims at providing a novel blood flow velocimetry tool using an ultrafast X-ray probing and inexpensive digital subtraction angiography (DSA) that can recover precise velocity distribution inside of the vascular systems especially for complex geometries. The long-term goal of the proposed research is to provide critical blood flow characteristics in vascular systems allowing scientists to predict the formation, evolution and failure risk of vascular pathology such as arteriovenous malformations, arterial occlusions, stenosis, and aneurysms. The proposed technology has a great potential in generating a real-time perioperative assessment of the blood velocity during a DSA routine. The novel technology will enable medical scientists to gain more fundamental knowledge about the nature and behavior of hemodynamics in cerebrovascular systems. The project is highly relevant to NIH's mission because the precise real-time assessment of blood velocities will lead to more educated therapeutic decisions which could save more lives, improve health, and reduce operation cost. The expanded knowledge base will enhance the Nation's economic well-being and ensure a continued high return on the public investment in research.